Full color information systems for the display of high definition video information and complex pictorial and graphics images must (1) provide high image resolution for precise edge definition and image sharpness; (2) maintain high display and image luminance and contrast control for maximum image brightness in a variety of display environments; and (3) achieve high color fidelity by maintaining precise, predictable control over color synthesis and reproduction using the largest color gamut available. For purposes of the discussion herein, the term "full color" display or display image means a display or display image which is capable of producing color from the full spectrum of visible light, and which uses at least three additive or subtractive primary colors to produce the full spectrum. In addition, full color, high information content display systems must also be (4) small and compact; (5) should minimize power consumption; (6) should maximize response time (the speed with which the display can be updated); and (7) must be able to be manufactured at a low unit cost in order to be useful in a wide variety of applications.
Existing color displays using a variety of technologies to produce full color, high resolution displays are deficient in one or more of the goals enumerated above. For example, the dominant color production system for the production of direct view, full color visual displays utilizes shadow-mask cathode ray tube (CRT) technology. While CRT displays have sufficient image resolution and generally good color rendition, they are typically large and heavy, consume considerable power, and have marginal luminance and contrast for the variety of environmental conditions in which displays must operate.
1. Twisted Nematic AMCLCD Technology.
A leading alternative to CRT display technology for high information content, full color visual displays is backlit liquid crystal display (LCD) technology. Displays using liquid crystal technology are thin and light, and offer potentially compact construction. Further, voltages and power levels required to actuate liquid crystal materials to a preferred orientation for modulating incident light are relatively low. LCD technology for high information content displays can be divided into two categories: nonactive multiplexed matrix technology and active matrix addressed color liquid crystal display (AMCLCD) technology. In nonactive multiplexed matrix technology, pixel positions are energized by voltages applied via intersecting row and column drivers. This is usually done by multiplexing the rows and turning on the needed column drivers in synch with each row as it is multiplexed. Displays with multiplexed LCD panels are limited in the image resolution which they may achieve, and have generally low contrast. Further, image quality, chromatic integrity, luminance and contrast are significantly affected by the angle at which the observer views the display. More importantly, displays with multiplexed LCD panels have inherently slow response times which are currently inadequate for supporting the display of output generated from graphics (animation) and pointing device (mouse) software.
In AMCLCD technology, the matrix of liquid crystal pixels consists of an active area (a region where the liquid crystal can modulate the input light) and an inactive area (a region composed of electrode data and scan lines and busbars, which are collectively called "interconnect"). The active area of each picture element (pixel) is controlled by an active element, usually a thin-film transistor (TFT), located at each pixel position. Such a matrix structure of active thin-film transistors can be fabricated with the deposition of a suitable material for forming thin-film transistors which typically includes, but is not limited to, polysilicon, amorphous silicon, semiconducting, and insulating films. These films are patterned using standard lithographic techniques to form the desired structure. The transistors are arranged in a matrix and deposited on a transparent substrate to form a planar matrix of sample-and-hold elements with associated transparent electrodes, each element being individually addressed by row and column in much the same way as a bit in memory. A planar body of liquid crystal material is sandwiched between a planar transparent common conductive electrode and the planar matrix of sample-and-hold elements with associated transparent electrodes, so that a field may be established across pixel elements of the liquid crystal. The state of an individual pixel of the liquid crystal display panel is controlled spatially by the application of a suitable voltage to gate lines and source lines to energize a locally addressable transparent electrode corresponding to the pixel within the matrix. The voltage can be placed and stored at each transparent electrode by the active thin film transistor.
The liquid crystal material which provides response to stimulation at video rates, and which provides the best ratio of the ON to OFF state (which is the major factor in determining display contrast) is nematic liquid crystal material. In thenematic structure, the direction of the long axes of the molecules in a given layer are slightly angularly displaced from the direction of the molecular axes in an adjacent layer. By applying a twist to this molecular packing, a helical structure is formed, and this mode is known as twisted nematic liquid crystal. Since the individual liquid crystal molecules have an elongated shape and dipoles (both permanent and induced) which are direction dependent, films of these materials exhibit anisotropy in their dielectric constant and refractive index. Materials that exhibit a positive dielectric anisotropy have molecules that tend to align themselves parallel to an applied electric field, while the molecules of materials that exhibit a negative dielectric anisotropy tend to align themselves perpendicular to the field. Because of their optical anisotropy, a change in the orientation of the liquid crystal molecules by an electric field can cause a change in polarization of transmitted light and thus a change in optical transmission when used in conjunction with light polarizers.
A transmissive, twisted nematic liquid crystal device 10, hereafter referred to as a "TNLC cell", is shown in FIG. 1A in the off state and in FIG. 1B in the energized state. Reference will be made to the location of components in TNLC cell 10 according to their relationship to light source 8, components closest to light source 8 being at the "rear" of TNLC cell 10, and components located farthest from light source 8 being at the "front" of TNLC cell 10. TNLC cell 10 comprises parallel, rear and front transparent substrates, respectively 13A and 13, each having transparent electrode layers 16A and 16, respectively, on the inner confronting surfaces of the substrates. Individual transparent elements on one of the electrode layers 16 or 16A, shown in FIGS. 1A and 1B on electrode layer 16A, are addressed by the matrix of active elements of the type described earlier, which controls the amount of voltage applied to each element. A thin layer of liquid crystal material of the nematic type is sandwiched between the substrates, and transparent alignment layers (not shown), which cover the transparent electrodes 16, maintain the liquid crystal molecules 14 in a fixed orientation relative to the substrate surfaces 13A and 13 while the molecules are in their stable relaxed state, i.e., with no voltage, or when a voltage below a threshold voltage is applied.
Optical control of light transmission from light source 8 is achieved by placing a rear polarizer 11 where the light enters the optical path of the liquid crystal display and another, front, polarizer (or analyzer) 12 where the light exits the optical path of the display. Both polarizers may be attached to the respective transparent substrates. For purposes of illustration, FIG. 1A shows that the polarizing axes of polarizers 11 and 12, denoted by arrows 15 and 20 respectively, are parallel. Rear polarizer 11 linearly polarizes the light incident to the TNLC cell in a first polarization plane, or orientation, indicated by arrow 15. Because of the relatively gradual twist of the liquid crystal molecules, polarized light entering the liquid crystal material gradually twists its direction of polarization as it passes therethrough, emerging with its polarization rotated by 90 degrees, to a second polarization orientation. The incident light in the second, orthogonal polarization orientation, is blocked by exit polarizer 12, resulting in a dark or opaque TNLC cell when viewed from the output side 20, with a substantial amount of polarized light being blocked by the polarizing direction of analyzer 12.
With the application of an alternating current (AC) voltage from source 21 that is above a certain threshold applied across electrodes 16 and 16A, as shown in FIG. 1B, the molecules 14 of the TN material rotate to align themselves substantially parallel to the applied electric field. In this energized, electrically driven, stable state (or ON state), the liquid crystal layer no longer twists the direction of the polarization of the light, and the light is transmitted by analyzer 12. At intermediate voltages, partial light transmission occurs. It is well understood by those skilled in the art that when polarizers are used on either side of the TNLC cell to control optical transmission, their transmission axes can be either parallel or perpendicular, depending on whether it is desirable that the driven (ON) state of the TNLC cell transmit light or block light, respectively. Hence, if the transmission axis of the exit polarizer 12 is parallel to that of the entrance polarizer 11, the TNLC cell will be dark in the absence of an applied voltage but will become transparent with the application of the voltage. Likewise if the transmission axis of the exit polarizer 12 is perpendicular to that of the incident polarizer 11, the TNLC cell will be transparent without a voltage and become dark with the application of a voltage.
Large format, high resolution active matrix displays using TN material have good display contrast and can achieve response times fast enough to accommodate software animation and pointing device applications, thus making them favored replacements for displays using CRT technology because of their packaging and luminance advantages.
2. Display Luminance
Display and image luminance, luminous efficiency, and color fidelity are interrelated factors in achieving a high quality, full color display using AMCLCD technology. Many LCD displays are perceived as dim in many ambient lighting situations. In a transmissive mode of operation, the display luminance is determined by both the luminance of a backlight, placed behind the display, and the transparency of the display. One factor which affects the transparency of the display is the structure of the active matrix addressing elements comprising the transparent conductive electrode. The transparency of the display is normally reduced by the amount of inactive area in the matrix, i.e., the presence of the opaque active elements and interconnect within the pixel. Since the transparent conductive electrode may comprise an area from about thirty per cent (30%) to about eighty per cent (80%) of the area of each pixel, depending on the display application, considerable display luminance may be lost by light which is blocked by the opaque addressing elements and which never enters the transparent portion of the pixel.
Another factor which affects display luminance is the use of color filters for color rendition. A predominant color synthesis method used in direct view, full color visual displays is additive color synthesis based on the spatial juxtaposition, or spatial proximity, of primary colors; the human eye perceives a color by spatially integrating the very small, juxtaposed primary (e.g., red, green, and blue) color pixels of the display. Additive spatial proximity color synthesis requires high pixel density (resolution) because the projected angular subtense of the primary color elements must be encompassed within the spatial integration zones of the human visual system in order for the eye to integrate a set of individual primary color pixels into a single mixture color.
For full color image formation, a matrix of active, individually addressable liquid crystal pixel elements or light valves (LCLVs) is typically constructed with a mosaic of individual (narrow band) primary color filters arranged on a single layer. The color filter mosaic is positioned immediately adjacent to the liquid crystal panel such that each individual color filter is in registration with a respective individual pixel element or LCLV and transmits only the desired portion of broadband (white) light while absorbing all other wavelengths. The backlight in such a display is typically a broadband light source with associated optical elements for controlling the spatial and angular distribution of the light before it enters the LCD panel. The absorptive color filter mosaic is a highly inefficient use of the energy contained in the light source, since it absorbs a substantial amount of the incident white light, often as much or more than one-half, during color selection and consequently reduces the luminance of the display still further.
Efficient generation and control of the radiation emanating from the backlight of the display therefore affords a practical and viable opportunity for maximizing the overall perceived brightness and luminous efficiency of the display. One solution, in simplest terms, is to increase the luminance of the backlight, and consequently the display, by increasing its power consumption. However, this introduces additional undesirable effects. In addition to conflicting with the goal of a display with low power consumption, this may produce harmful temperature rises within the system.
3. High Efficiency Color Emitting Backlight Source
Generating color without the use of light-absorbing filters provides an efficient solution to the display luminance problem. A backlight source composed of a patterned matrix of individual primary colored phosphor emitters in the same spatial configuration as the matrix of liquid crystal pixel elements in the LCD panel eliminates the need for the light-absorbing color filters, thereby providing more light directly into and through the liquid crystal light valves. Any suitable energy source may be used which provides either ultraviolet radiation or electrons for exciting the primary colored phosphor emitters to emit visible primary colored light.
A backlight source which emits primary colored light in a spatial matrix and which has the same spatial configuration as the matrix of pixel elements is described in U.S. Pat. No. 4,799,050 issued to Prince, et. al., entitled "Full Color Liquid Crystal Display". The light source described therein is suitable for use in the present invention. FIG. 2 illustrates the light source, comprised of a single or multiple lamps 30 and associated optical components, collectively designated as light source 22. Fluorescent lamp 30 is positioned at the backplane of the liquid crystal light valve disclosed in U.S. Pat. No. 4,799,050. The output of lamp 30 comprises ultraviolet radiation 33, rather than visible light, and fluorescent lamp 30 is chosen for its ability to excite phosphor material to emit visible light Fluorescent lamp 30 is also chosen for its intrinsically high energy conversion efficiency of fluorescent illumination, exceeding 50 lumens per watt. The electrodes of lamp 30 are enclosed within a clear envelope of quartz or other ultraviolet-transmissive material, and plate 32, which forms the rear surface of the light valve, is also preferably quartz and transmits ultraviolet radiation.
A first dichroic selective filter layer 34, which may be deposited on the inner surface of plate 32, is fabricated of material that is transmissive to ultraviolet radiation and reflects visible light. Ultraviolet radiation 33 from lamp 30 is thus transmitted through intervening layers 32 and 34 to reach layers 36 and 38.
Next in alignment, and closely spaced to first dichroic filter 34 is a second dichroic filter 38 which has deposited on it, on the surface closest to first dichroic filter layer 34, a phosphor layer 36 comprising a matrix arrangement of a plurality of discrete phosphor elements. The phosphor layer includes any material that converts energy from an external excitation and, by means of the phenomena of phosphorescence or fluorescence, converts such energy into visible light. In the embodiment of the LCD disclosed in U.S. Pat. No. 4,799,050, the dominant emission spectrum of fluorescent lamp 30 is between 254 nm. and 365 nm., a spectrum selected to match the excitation spectrum of the phosphors. Other energy sources, including electron beams are known to excite phosphors to emit visible light and may be substituted for fluorescent lamp 30.
In the embodiment of the LCD disclosed in U.S. Pat. No. 4,799,050, each phosphor element in phosphor layer 36 is aligned with a corresponding transparent pixel element in an active electrode matrix such as matrix 16A, shown in FIG. 1 and described above in the discussion of AMCLCD technology. Active electrode matrix 16A controls the transmission of light through the display. The alignment of the phosphor elements with respective pixel elements is accomplished by a light aligning technique described more fully below. The phosphor and electrode matrices may be formed onto a common material, but need not be in the present invention. In the case of a full color display, the phosphor element matrix consists of a plurality of phosphor types, each of which emits a wavelength emission spectrum in one of the primary colors upon excitation by ultraviolet radiation. The plurality of phosphors is preferably arranged into a pattern for integration by the eye into a single color from a mixture of primary colors by additive spatial juxtaposition. This pattern may include, for example, the conventional triad, quad (in which the green component is doubled) or fixed format geometries. The phosphors may be deposited by any of a number of well known processes including settling, screen printing, and photolithography.
As radiation 33 reaches the matrix of phosphor elements in phosphor layer 36, the ultraviolet radiation is efficiently converted to colored visible light. Ultraviolet radiation not initially absorbed by the phosphors in phosphor layer 36 is reflected from second dichroic layer 38 and may thereafter by absorbed by the phosphors, further increasing the efficiency of the system.
The visible light 40 emitted by the phosphors is radiated toward both the front and back of the liquid crystal light valve (toward plate 32 and dichroic filter 34). Due to the visible light reflectivity of the dichroic filter 34, the phosphor emissions that travel toward the backside of the light valve (not shown in FIG. 2) are reflected therefrom and toward the front of the display (as defined from the perspective of the observer). The close spacing between the phosphor matrix 36 and the adjacent dichroic filter 34 allows only minimal "spreading" of the reflected phosphor outputs. Thus, a high percentage of this reflected light is available for illumination of the display.
According to U.S. Pat. No. 4,799,050, use of fluorescent lamp 30 in conjunction with the color phosphor matrix, and in the absence of the matrix of color filters typically used in full color liquid crystal displays, achieves the maximum available energy efficiency offered by the utilization of fluorescent energy. This, in turn, directly results in a brighter display because substantially all of the generated light is employed.
For this highly efficient lighting approach to be effective, however, the colored light 40 from each phosphor emitter in phosphor layer 36 must be directly coupled to and channeled into a respective liquid crystal pixel element, and the channeled light must pass though only one pixel of the light valve. If the colored light from one emitter is allowed to spread and pass through an adjacent pixel, the chromatic integrity of the display will be negatively affected.
One approach to accommodate the requirement of carefully channeling the light from the phosphor emitters is described in U.S. Pat. No. 4,799,050. In this approach, the colored light sources are placed on one side of a fiber optic plate and the aligned active matrix with associated transparent electrodes is placed on the other side. The fiber optic plate channels the light and thus maintains the spatial distribution of the emitted light, preventing the light emitted from one pixel from spreading out and passing through other pixels. U.S. Pat. No. 4,799,050 discloses that the light emitters and the active matrix can be built on separate fiber optic plates which are optically bonded during assembly.
There are two main disadvantages in using a fiber optic face plate for the purpose of channeling the colored light from the phosphor emitters through the corresponding pixels in the light valve. The first is a fabrication problem. Using a fiber optic face plate adds significantly to the final unit cost of the display. In addition, the lithographic steps utilized in building a liquid crystal display on the fiber optic face plate require a very flat substrate material, and some of the processing steps require the substrate to retain this flatness after being subjected to high temperatures. The manufacturing complexity added to the fabrication of the display from using the fiber optic face plate decreases the reliability and significantly increases the unit cost of the display, which conflicts with the goals enumerated above for achieving a high quality, full color display.
More limiting than the manufacturing complexity, however, is the fact that the use of the fiber optic face place in the manner described in U.S. Pat. No. 4,799,050 makes more complicated and difficult the incorporation and operation of the entrance polarizer needed for use with TN liquid crystal material. Typically, a thin entrance polarizer is utilized in conjunction with TN liquid crystal material. Conventional TNLC configurations only change the state of light polarization. Polarizers are required to transform the light polarization changes to light intensity differences which are detectable by the human eye. Film polarizers, commonly utilized because of their low cost, have thicknesses on the order of 150 to 400 microns. If the colored light from the phosphor emitters is polarized by the entrance polarizer after being channeled through the fiber optic face plate, the spatial differentiation of the emitted light created by the fiber optic face plate will not be maintained as it passes through the polarizer. Instead, the light will spread in proportion to the thickness of the polarizer, the magnitude of such spreading depending on the divergence of the rays at exit from the optical fibers. This defeats the light channeling effect of the fiber optic face plate. Thus, a conventional polarizer could not be used effectively in conjunction with the fiber optic face plate, and a more expensive, potentially more fragile polarizer would have to be fabricated as an extremely thin film to reduce the spreading of the polarized light to adjacent pixels. In high resolution displays, the thickness of the polarizer would have to be less than the pixel size, which is in the range of approximately 50 to 150 microns, to prevent the colored light from illuminating adjacent pixels.